Torsion bars are used in many applications to provide spring resistance where space is limited such as in vehicle suspension systems. In a typical torsion bar vehicle suspension system, a torsion bar is attached at one end to each front wheel. At the wheel end of the torsion bar, a control or radius arm is connected, extending radially from the longitudinal body or spring portion of the torsion bar. Although the spring portion and radius arm may be formed as separate parts which are then joined together, it is usually desirable to form the spring portion and radius arm integrally by bending a single rod. As used herein, "Torsion bar" will include its cooperating radius arm unless otherwise specified. The spring portion of the torsion bar is connected to the frame of the vehicle with a turning anchor adjacent the wheel end and solid anchor at the end opposite the wheel end. The torsion bar can rotate within the turning anchor, but is rigidly retained in the solid anchor. Hence, movement of the radius arm applies a torque to the torsion bar when the vehicle wheel rises. Since one end of the torsion bar is rigidly held by the solid anchor, the torsion bar twists in response to the applied torque. Therefore, the torsion bar must be made of a material that offers spring resistance to the twist such that it yields to the torque and then untwists in a rebound action.
Generally, torsion bars used in vehicle suspension systems are formed from a uniform solid rod of metal. Such torsion bars can be easily fabricated and do not require heat treating. However, the emphasis in recent years on reducing vehicle weight has made it desirable to reduce the weight of all parts, including torsion bars. It is known that a one-piece torsion bar is weakest at the location of any bends in the bar. Stresses are concentrated at the bends similar to the effect of a notch in a beam. When a twist is applied to the bar, the twist forces are concentrated at the bends. These stress points at the bends in the torsion bar made it necessary to use high-strength steels or heat treated steel to prevent cracking.
With the goal of weight reduction in mind, others have attempted to fabricate torsion bars from hollow tubes. One disadvantage of hollow torsion bars currently being evaluated is that the hollow tubes must be made of expensive premium grade steel such as for example a high strength, low allow (HSLA) steel. Another disadvantage in the use of hollow torsion bars is the necessity of heat treatment to improve the bar strength after it is cold formed or forged in the shape of the torsion bar. When a torsion bar is heat treated after forming, close dimension tolerances often cannot be met due to thermal distortion of the torsion bar. Heat treating also adds considerably to the cost of producing hollow torsion bars. Due to the degree of bending required to form a one-piece torsion bar, the tube must be strong enough to prevent collapse of the tube. Alternatively, a mandrel may be required to prevent the tube from collapsing during the bending process. It is also known that a hollow tube, if unreinforced, lacks vibrational stiffness which may produce unwanted acoustic resonance if the natural frequency of the hollow tube is too close to the resonant frequency of the drive train.
Other attempts to develop a functional hollow torsion bar include providing aluminum cores at the critical bends such as the angle at the solid anchor and the angle which joins the radius arm and spring portion of the torsion bar. However, use of an aluminum core at a bend provides only limited weight savings and requires a complex assembly process. Typically, the aluminum core must be press fitted within the tube. If the core dimensions do not precisely match the tube bore, the tube may split when bent or it may be impossible to insert the aluminum core into the tube. If the aluminum core is too small, it may be displaced since the aluminum core is not bonded to the tube. Also, it is known that placing aluminum cores in steel tubes causes accelerated corrosion due to the dissimilarity of the metals.
The use of rubber torsion bars has also been explored by others. These rubber torsion bars have a outer steel tube or shell and an inner steel tube or sleeve which comprises the spring portion. A rubber liner is provided between the shell and sleeve in a complex manufacturing process. While some bend strength is gained, the bar weight is increased by the use of two steel tubes and dimensions must be precise.
Therefore, it is evident that one object in the design of torsion bars is to provide a light weight, one-piece torsion bar. While a hollow torsion bar is lightweight, it must still provide strength equivalent to solid torsion bars in both compression strength and shear strength. A suitable hollow torsion bar must meet current performance specifications for solid torsion bars. A further object is to provide a torsion bar which does not suffer from excessive vibration and unwanted resonance characteristics. Of course, a suitable hollow torsion bar must be able to withstand the shocks and twisting forces exerted upon it without fracturing or loosening.
Another objective in the development of torsion bars is that the torsion bar must be simple to manufacture by a reliable process. A practical hollow torsion bar must be cost competitive with solid torsion bars and preferably less expensive than solid bars. It would also be desirable to avoid the use of expensive, high strength steels. A further objective is to eliminate the need for heat treatment to meet strength specifications. The present invention achieves these goals.